autoreload_test: Handle a relative PYTHONPATH

This came up in the Debian package build of tornado, where we run the
tests from a staged build of the module.

tornado/test/autoreload_test.py

@@ -73,9 +73,13 @@ def write_files(self, tree, base_path=None):
  def run_subprocess(self, args):
  # Make sure the tornado module under test is available to the test
  # application
- pythonpath = os.getcwd()
+ parts = [os.getcwd()]
  if "PYTHONPATH" in os.environ:
- pythonpath += os.pathsep + os.environ["PYTHONPATH"]
+ parts += [
+ os.path.join(os.getcwd(), part)
+ for part in os.environ["PYTHONPATH"].split(os.pathsep)
+ ]
+ pythonpath = os.pathsep.join(parts)
 
  p = Popen(
  args,